Tiny radio-powered stickers can measure forces between objects

In many situations, it's important to know how much force one object is exerting on another. We've designed a small, thin sticker to measure and transmit this type of data without the need for wires or batteries. The device, called ForceSticker, is being developed by scientists at the University of California, San Diego. Among other uses, it could be used to monitor the weight of stacked packages in a warehouse, ensure that artificial knee joints don't injure adjacent cartilage, or even provide robots with a sense of touch.

ForceSticker attached to the interface of two touching objects. Image/David Baillot/UC San Diego Jacobs School of Engineering

At its core, ForceSticker is a capacitor about the size of a grain of rice. It consists of a soft polymer sheet sandwiched between two conductive copper strips. When force is applied to the sticker, the polymer compresses, bringing the two copper strips closer together. This increases the charge on the capacitor.

Another major component of ForceSticker is an RFID (radio frequency identification) tag that is temporarily powered by a radio signal from a handheld RFID reader. The tag uses an integrated antenna to transmit a modified signal back to the reader - a signal that now contains information about the current charge level of the link capacitor. Custom software then converts this data into a measurement of the external force.

Researchers successfully tested ForceSticker in a knee joint modelDavid Baillot/UC San Diego Jacobs School of Engineering

By varying the hardness of the polymer used in the capacitor, the ForceSticker can be made to be very sensitive, suitable for measuring small forces, or less sensitive, suitable for measuring larger forces. In current testing, the highly sensitive sticker was used to monitor forces on a model knee joint, while the less sensitive sticker was used to monitor the weight of various large objects placed in a box.

In both cases, the ForceStickers were still effective after more than 10,000 force applications. The stickers are also relatively cheap to make, costing about $2 each, a figure that should drop significantly if production is scaled up. In addition, users will eventually be able to use their smartphones instead of dedicated RFID readers.

"Humans are born with the ability to sense force. Providing this force sensing capability to electronic devices and medical implants could be a game changer for many industries," said lead scientist Professor Dinesh Bharadia.

A paper on this research will be presented at the UbiComp 2023 conference in Mexico.